Datasheet

TPS40055-EP

SGLS310D –JULY 2005–REVISED FEBRUARY 2012

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The Q

is not always described in a MOSFET's data sheet, but may be obtained from the MOSFET vendor.

The total synchronous rectifier MOSFET power dissipation is described in Equation 39.

(39)

TPS40055 POWER DISSIPATION

The power dissipation in the TPS40055 is largely dependent on the MOSFET driver currents and the input

voltage. The driver current is proportional to the total gate charge, Q

, of the external MOSFETs. Driver power

(neglecting external gate resistance)

[2]

can be calculated from Equation 40.

(40)

And the total power dissipation in the TPS40055, assuming the same MOSFET is selected for both the high-side

and synchronous rectifier is described in Equation 41.

(41)

(42)

where:

is the quiescent operating current (neglecting drivers)

The maximum power capability of the device's PowerPad package is dependent on the layout as well as air flow.

The thermal impedance from junction to air, assuming 2 oz. copper trace and thermal pad with solder and no air

flow.

(43)

The maximum allowable package power dissipation is related to ambient temperature by Equation 44.

(44)

Substituting Equation 45 into Equation 41 and solving for f

yields the maximum operating frequency for the

TPS40055. The result is described in Equation 45.

(45)

LAYOUT CONSIDERATIONS

PowerPAD™ PACKAGE

The PowerPAD package provides low thermal impedance for heat removal from the device. The PowerPAD

derives its name and low thermal impedance from the large bonding pad on the bottom of the device. For

maximum thermal performance, the circuit board must have an area of solder-tinned-copper underneath the

package. The dimensions of this area depends on the size of the PowerPAD package. For a 16-pin TSSOP

(PWP) package, dimensions of the circuit board pad area are 5 mm x 3,4 mm

[2]

. The dimensions of the package

pad are shown in Figure 17.

Product Folder Link(s): TPS40055-EP